Thin filmed materials are often utilized in semiconductors, ceramics, metals and superconductors. For example, thin layer materials are utilized in the production of highly complex silicon integrated circuits. A number of methods are known for producing thin films including vacuum evaporation, molecular beam epitaxy, and atomic layer epitaxy (ALE), etc. Additionally, such materials are useful in the development of electronic devices based upon bandgap engineering.
Particularly, atomic layer epitaxy has been used for growth of compound semiconductors and other thin film structures. C. H. L. Goodman and M. V. Pessa, Atomic Layer Epitaxy, Journal of Applied Physics 60(3) , Aug. 1, 1986, R65-R81; M. Ozeiki et al., New Approach to the Atomic Layer Epitaxy of GaAs Using Fast Gas Stream, Applied Physics Letters, Vol. 53, Number 16, Oct. 17, 1988, pp. 1509-1511; U.S. Pat. No. 4,058,430 to Suntola et al.. However, such growth has only been achieved with ionically bonded compounds. The growth of other than ionically bonded compounds by ALE has not been successful. Specifically, layer-by-layer growth of materials from non-elemental sources (gaseous chemical compounds) can not be achieved using ALE.
ALE was developed for the growth of ionically bonded materials, such as compound semiconductors. In its present form, ALE has only successfully been used for compound semiconductor growth. ALE achieves monolayer-by-monolayer growth by chemisorption of one gas phase component onto a saturated surface having a monolayer coverage of the other component. The reaction proceeds until the surface is saturated with the one gas phase component. At this point the saturation cover is believed to be only a monolayer thick because once the strong chemisorption bonds are saturated, physisorption is too weak to build up further layers. The role of the two components is then exchanged and the reaction is repeated. A film of finite thickness is built up by alternating the exposure of the surface to the constituent components.
ALE growth is conducted at growth conditions collectively known as the "ALE processing window", which is arrived at by systematic empirical study. Generally, the only analytical feedback on layer control in ALE is provided by external, remote analysis of the grown films. In other words, the article must be removed from the process chamber and place in a separate and distinct testing device. The exposure times of the growth surface are then readjusted until this trial and error procedure produces a material that consists of alternating monolayers of constituent elements. The procedure for establishing the "ALE processing window" for compound semiconductor growth cannot be directly extended to metals or semiconductors with covalent bonding. In order to produce layer-by-layer growth materials from other than ionically bonded materials from gaseous molecular sources, a different controlling mechanism must be utilized.
Another disadvantage of existing ALE technology is that ALE is limited to binary systems that can simultaneously exhibit strong chemisorption and weak physisorption at temperatures appropriate for thin film growth. This constraint has limited the application of ALE to ionically bonded compound semiconductors.
Further, the static nature of ALE is determinative of its capabilities. Therefore, only a fully saturated surface satisfies the requirements for controlled growth.
A further disadvantage of existing ALE technology is that film growth in ALE occurs per cycle of exposure. Frequently one cycle of exposure is referred to as a pulse. One cycle of exposure produces a complete atomic layer of material.
Pulsed supersonic jets have been used to provide rapid deposition of a thin film upon heated substrates for the preparation of semiconductors and similar electronic devices. While the pulsed supersonic jet process grows a large amount of deposits in a short time, there is difficulty in controlling the process without feedback data.
As demonstrated by the patents to Hall (U.S. Pat. No. 5,009,485), Aspnes et al. (U.S. Pat. Nos. 4,931,132 and 4,332,833), Siegmund et al. (U.S. Pat. No. 4,878,755), Keller et al. (U.S. Pat. No. 4,846,920), Hartley (U.S. Pat. No. 4,770,895), Tien (U.S. Pat. No. 4,713,140), Strand et al. (U.S. Pat. No. 4,676,646), and Cole (U.S. Pat. No. 4,582,431), the use of optical methods for monitoring the development of materials is known in the art.
The disadvantages and limitations of the prior art techniques discussed above indicate the need for an efficient, versatile, and controllable method and apparatus for layer-by-layer production of thin films. The subject invention provides such a method and apparatus.